1. Field of the Invention
The invention relates to a control system for controlling an AC motor, and more particularly to a motor control system that converts DC voltage into rectangular wave AC voltage by means of an inverter, and applies the AC voltage to the AC motor.
2. Description of the Related Art
A motor control system is generally used which converts DC voltage into AC voltage via an inverter and applies the AC voltage to the AC motor so as to drive the motor under control. In the motor control system of this type, motor current is generally controlled according to sinusoidal pulse width modulation (PWM) control based on vector control, so as to drive the AC motor with high efficiency.
With the sinusoidal PWM control, however, a fundamental component of the output voltage of the inverter cannot be sufficiently raised and the voltage utilization factor is limited, which makes it difficult to provide high output or power in a high-speed region. In view of this point, it has been proposed to employ a control method that enables the inverter to produce an output voltage having a larger fundamental component than that produced under the sinusoidal PWM control.
Japanese Patent Application Publication No. 2006-320039 (JP-A-2006-320039) describes a control method according to which a rectangular wave voltage having an amplitude equal to that of voltage variably controlled by a converter is applied to the AC motor. In the control method described in JP-A-20006-320039, in particular, the voltage phase of the rectangular wave voltage is basically changed in accordance with a torque deviation, and, upon a rapid change of the rotational speed of the motor, the output voltage of the converter is changed in accordance with the rate of change of the motor speed.
In a drive control system for controlling an AC motor that is rotated with a rectangular wave voltage applied thereto, as described in Japanese Patent Application Publication No. 2006-54995 (JP-A-2006-54995), a first voltage phase determined according to an actual torque value based on detected voltage/current values and a command torque value, and a second voltage phase determined according to an estimated torque value based on a motor model and the command torque value are calculated, and a value obtained by weighing the first and second voltage phases is used as a phase of the rectangular wave voltage.
In the control systems of JP-A-2006-320039 and JP-A-2006-54995, phase control of the rectangular wave voltage is performed by feeding the actual or estimated torque of the AC motor back to the system. When a torque command value is changed during the torque feedback control, a torque deviation (a difference between the actual torque and the torque command value) due to the change in the torque command value is detected, and then the voltage phase is changed according to a control operation or computation for eliminating the torque deviation. Accordingly, there is room for improvement in the control response when the torque command value is changed.
In the meantime, the output torque of the AC motor is changed not only depending on the voltage phase as a manipulated variable of the rectangular wave voltage control, but also depending on motor operating conditions, a typical example of which is the motor rotational speed. Generally, the output torque changes non-linearly with respect to the voltage phase in the rectangular wave voltage control: therefore, when feed-forward control based on the torque command value is put into effect, a complicated operation (computation) and/or a high load on the system may be needed to calculate an amount of change in voltage phase corresponding to the required torque compensation amount.